“Trou au Natron” seems to translate from Fr*nch to “salt pit”. It’s probably a crater full of salt from an evaporated sea.

I should have just !wiki’d it. https://en.m.wikipedia.org/wiki/Trou_au_Natron

I was partially correct. Natron salt in a volcanic caldera from active hot springs.

The red marker on the original image probably is the site of this X shaped marking on the top of the western dark region of the “skull” - OP probably assumed the X was the four edges of a pyramidal shape but this 3d Google Earth view from the western “eye” toward the eastern one shows that the X shaped region is flat and not pyramidal.

Edit: probably someone did some mining up there? Or a natural feature? Who knows, let me know if you go check for sarcophagi to prove me wrong